SLI antialiasing modes exposed
I used the exceptionally handy Direct3D FSAA viewer to capture the sample patterns for the antialiasing methods available in SLI, Quad SLI, and ATI's CrossFire. The top four rows of the table show the 2X, 4X, 6X, and 8xS modes possible with single GPUs, as well. Below those are the SLI AA and CrossFire SuperAA sample patterns.

The yellow squares below represent the space inside of a pixel. The green dots represent texture samples, and the red ones represent coverage samples, used to determine polygon coverage in the multisampled antialiasing algorithms used by both Nvidia and ATI. (If multisampling is unfamiliar to you, I suggest reading this helpful article for an overview of it.)

	GeForce 7900 GTX	GeForce 7950 GX2 SLI	Radeon X1950 CrossFire
2X
4X
6X
8xS
8X
10X
12X
14X
16X
32X

There's much to discuss in the table above. Let's start with the differences between the GPUs, the Radeon X1950 XTX and the Nvidia G71. The Radeon hardware is more flexible; its sample patterns are fully programmable, and the Radeon can capture six coverage samples alongside just one texture sample on a single GPU. (Texture samples have a high performance cost, so avoiding them is helpful.) The Radeon's excellent 6X AA mode also uses a pseudo-random, non-grid-aligned sample pattern that ought to disrupt the eye's pattern recognition better than the rotated grid patterns used in both brands' 2X and 4X modes. ATI makes further use of its hardware's programmable sample patterns in the CrossFire SuperAA modes, where the pseudo-random sample distribution gets taken up a notch.

Since each Radeon in a CrossFire config must capture one texture sample, ATI gives the user a choice between its 8X/10X and 12X/14X modes. In 8X and 12X, the two texture samples are superimposed over one another, so there's no element of supersampling in the scene—only edges are modified, and none of the interior detail "blurring" that comes from supersampling is present. The 10X and 14X modes offer the same number of samples as their respective peers, but the texture samples are situated at opposite corners of the pixel. These modes compromise crispness for an element of full-scene spatial AA.

Generally, given the choice, I would side with the 8X and 12X sample patterns. Interior details like textures are best handled via anisotropic filtering.

The Nvidia GPU, on the other hand, has largely fixed sample patterns. The 4X pattern, in particular, is the basis for all of the GeForce's higher sample modes. Even the SLI AA modes use jittered versions of the 4X and 8xS patterns. The 8xS pattern is two copies of the 4X pattern, one above the other, squished into a single pixel. In order to get to sample sizes beyond four with a single GPU, Nvidia resorts to capturing two texture samples—a bandwidth-intensive task.

The G71 GPU's less capable hardware limits what Nvidia can do with antialiasing. Still, Quad SLI brings with it three new SLI AA modes, distinct from the modes used in dual-GPU SLI. The 32X mode splits up the work as expected, with each GPU contributing an 8xS antialiased image at an its own unique subpixel offset. As I mentioned earlier, SLI 32X AA has eight texture samples and 32 coverage samples, for a mix of supersampling and multisampling. In order to fit this pattern into the space of a pixel, Nvidia's been forced to jitter the pattern only slightly from one GPU to the next. The result is four sets of eight samples that unfortunately don't vary from one another in position by very much.

The Quad SLI 16X pattern is, also as expected, simply four copies of the base 4X AA pattern—one from each GPU, with an offset. That makes this mode very much distinct from the SLI 16X mode in dual SLI, which involves two overlaid 8xS images. Nvidia has made an interesting choice here, situating the four texture samples tightly around the pixel center. It's a choice that I tend to like, for the same reasons that I prefer ATI's 8X and 12X SuperAA modes.

The Quad SLI 8X AA mode is a different kind of animal. Like the SLI 16X mode, its texture samples are grouped tighter around the pixel center than in its dual-GPU counterpart. The really odd thing here, though, is that the pattern is simply two copies of the base 4X AA sample pattern—not four copies of the 2X pattern, which would be what's expected in a Quad SLI AA mode.

I suspect Nvidia must be doing something different with its load-balancing here—a suspicion bolstered by the fact Quad SLI 8X AA has unexpectedly strong performance. Whatever's happening, it's apparently secret sauce to Nvidia. When I asked them what was going on with the Quad SLI 8X mode, they wouldn't tell me unless I would agree not to disclose it. My best guess is that they are running 4X AA on a pair of GPUs, resolving the resulting images to this 8X pattern, and then doing alternate frame rendering between GPU pairs. This method would sidestep the DirectX three-buffer limit and potentially scale well.

Performance expectations for Quad SLI
Nvidia has been extremely careful in slipping Quad SLI out into the market. They first told the world about it at CES early this year, and then Quad SLI systems began shipping via a select group of system builders like Falcon Northwest. The early quad-capable dual-GPU cards were never sold to the public, and were replaced by the GeForce 7950 GX2. Even after the GX2 arrived, PC enthusiasts who wanted to build their own systems had to wait until this past August to get their hands on official release drivers that would enable Quad SLI.

Even as those drivers were released, Nvidia sought to position Quad SLI as a targeted solution that will appeal to a very narrow segment of the market. Those select few, they argued, would want to run their games at extreme resolutions, like 2560x1600, with high degrees of anisotropic filtering and edge antialiasing. Here is a chart from Nvidia's Quad SLI reviewer's guide that lays it out for you.

Only the bright green need apply. Source: NVIDIA.

So they're being very upfront about the fact that Quad SLI isn't for everyone. Heck, if you read the reviewer's guide, we're probably not really qualified to review this product due to our puny 2048x1536 display! So you can just stop reading now, folks.

If you do choose to soldier on, you will see actual, empirical performance numbers for Quad SLI from a range of resolutions and edge and texture antialiasing settings. In fact, we've used our entire test suite from our recent graphics reviews to compare Quad SLI to a broad array of solutions. Some of these results may not show Quad SLI in the best light, but the important thing is to remember how Quad SLI makes you feel. That is, of course, the reason for spending all of that money.

In case you're not getting the hint, all of this caution and fine-grained product positioning ought to temper your performance expectations for Quad SLI. Nvidia says the CPU overhead of driving four GPUs is what causes Quad SLI problems at lower resolutions and quality levels, and I'm sure that's one part of the equation. Our test rig's new Core 2 Extreme X6800 processor is incredibly fast, though, and other factors figure prominently into the mix, as well—not least of which is the three-buffer limit in DirectX 9. Not being able to run in AFR mode has gotta hurt. Then there's the fact that the individual GPUs on the GeForce 7950 GX2 aren't the fastest on the market. They're each about the same speed as a GeForce 7900 GT or GS. Cards like the GeForce 7900 GTX or the Radeon X1950 XTX offer quite a bit more performance per GPU—and they can run in dual-GPU configurations, as well

Quad SLI may become more attractive when games that use Havok FX's GPU-based physics API finally arrive. Then, a quad setup could dedicate two or three GPUs to graphics and leave the rest to handle physics acceleration. At present, though, no Havok FX-enabled game titles or video drivers are available.

Of course, we've not only tested Quad SLI in our regular graphics performance suite. We've also done some extended testing at extremely high antialiasing and anisotropic filtering settings, and we've examined image quality, too.